The energy of the sun is converted into electricity by photoelectric cells on the solar array
Every satellite in space requires electrical power – power that comes from the sun. The solar panels which capture this are robust, with efficiency now reaching a formidable 29%. Over the past 40 years, Airbus Defence and Space’s experts have developed and built more than 450 flight units (generally consisting of two solar wings) with an output of a few hundred Watts up to 26 kW. To date, not a single one of these units has failed while in operation.
Power in space
A satellite needs electrical power to operate. One of the main purposes of the platform is to collect, store and distribute this power.
Nearly all satellites are powered by solar energy. They are equipped with a solar array, usually in the form of one or several panels, carrying a number of cells that directly convert sunlight into electricity. The first high-efficiency photoelectric (solar) cells were developed by Bell Laboratories of AT&T (USA) in 1954. These cells converted 6% of the solar energy they received into electricity. Today, this technology is very widely used, with silicon cells working at 12–17% efficiency, and gallium arsenide cells reaching up to 29%. This gives a total power output at beginning of life of up to 220 watts per square metre; but output decreases over time. Development of gallium arsenide technology is continuing and should boost conversion efficiency to more than 30% – but these advanced materials are more expensive.
The satellite’s solar panels are folded during launch to fit inside the launcher’s payload fairing. Solar array deployment is critical; if it doesn’t open, the satellite won't work and the mission is lost!
Where do satellites get their power during eclipse periods, when the satellite is in the Earth’s shadow?
Without sunlight, there’s no power – and therefore no TV broadcasts or phone calls for people down on Earth? To overcome this problem, and to provide extra power during peak consumption periods (especially for Earth observation spacecraft), satellites are equipped with battery systems. These batteries can be recharged in about 12 hours. But the frequent charge/discharge cycles typical of Earth observation satellites tend to limit the batteries’ useful life – and also the satellite’s!
A geostationary satellite experiences eclipses only during two near-equinox periods a year (March, September) and these eclipses last no more than 72 minutes a day, or 5% of the total time. For a satellite in low orbit, circling the Earth once every 100 minutes, eclipse periods may represent up to 40% of the total time, which means the batteries are used much more frequently.
Pointing the solar panels
A satellite’s solar panels must always point towards the Sun if they are to collect the greatest amount of energy. But the payload does not automatically share the same pointing requirements. Two different solutions are therefore used:
1) On spin-stabilised satellites, the entire bus is covered with solar cells, although only about one third will be effective at any given moment.
2) On 3-axis stabilised satellites, a drive mechanism always keeps the solar panels pointing toward the sun.
The satellite platform is also equipped with a set of power conversion and regulation units to control the storage of electrical energy and its distribution to the payload and service equipment.
Communications satellite: 920 cycles over 10 years
Earth observation satellite: 27,000 cycles over 5 years